TD-SCDMA belongs to the family of 3G wireless standards as defined by the 3GPP and will be deployed by many carriers in their TDD mode. The 3GPP has adopted two different chip rates for this mode, 3.84 Mcps high chip rate mode and 1.28 Mcps low chip rate version. The former is compatible with WCDMA and has frame durations of 10 ms with 15 slots. The latter has a frame duration of 5 ms with 7 slots/frame. Both versions of TD-SCDMA include sophisticated physical layer techniques such as smart antennas and joint detection to increase system capacity even with inter-chip and multi-user interference.
Because of the slotted TDMA scheme with short PN sequences and a small number of simultaneous users, multi-user detection (MUD) are now possible even in the downlink. Multi-user detection techniques for the uplink have been advocated even during the standardization process. See M. Vollmer, M. Haardt, J. Gotze, “Comparative Study of Joint-Detection Techniques for TD-CDMA Based Mobile Radio Systems,” IEEE J. Selected Areas of Communications, pp. 1461-1475, August 2001 (Vollmer et al. I). However, the complexity of such optimal detection techniques has prevented their widespread use especially on the downlink at the mobile.
Joint or multi-user detection at the baseband tremendously improves performance over a conventional Rake receiver. The latter deteriorates in performance when multipath reduces the orthogonality of the spreading codes. However, the disadvantage is the increased receiver complexity when compared to the more conventional, though sub-optimal Rake receiver. Several block-based multi-user detection schemes are investigated. Zero-forcing and MMSE techniques based on matrix factorizations have been studied. See, e.g., “Zero-forcing and minimum-mean square error equalization for multi-user detection in code-division multiple access channels,” IEEE Trans. on Veh. Tech., pp. 276-287, May 1996; and Vollmer et al. I.
The present disclosure is a method of multi-user detection in a given uplink and downlink time slot in a software-defined receiver. The method includes filtering and sampling a received signal; forming a block-banded matrix A of the sampled signals; and solving {circumflex over (d)}=T−1y, where T=(AHA), y=AHx.
A first method of solving for the matrix T is by applying the inverse matrix T of the system matrix T1 using a zero-forcing equalizer or decorrelating detector; and computing Cholesky factors of the matrix T by approximating using the block-banded property of the matrix T and A.
A second method is by Schur decomposition for Cholesky factors of the matrix T. The second method includes computing a generator matrix G of the matrix T; Givens rotating the matrix G to produce lower triangular Cholesky factor matrix R; and approximating matrix R using block Toeplitz property of matrix T. Also, only first couple of block columns of the matrix R are computed and the last block is copied.
A third method is by Fourier Transformation and includes padding the matrix T to make it circulant before solving.
These and other aspects of the present disclosure will become apparent from the following detailed description of the disclosure, when considered in conjunction with accompanying drawings.